Abstract: In this article, waste phoenix leaf biochar was introduced into three most used organic phase change materials (PCMs) to prepare shape-stable composite PCMs by the vacuum impregnation method. The encapsulation capacity and encapsulation efficiency of steam activated biochar for paraffin, stearic acid, and polyethylene glycol were 59.28% and 49.14%, 63.03% and 57.62%, 71.38% and 46.84%, respectively, which was related to the mechanisms of biochar encapsulated organic PCMs. The dominant mechanisms of biochar encapsulated organic PCMs are summarized as pore-filling, hydrogen-bonding, hydrophobic interaction between biochar and PCMs, and other potential mechanisms. Influence degree η was introduced to illustrate the effect of biochar on organic PCMs, 16.56%, 8.72%, and 35.75% for steam activated biochar on paraffin, stearic acid, and polyethylene glycol. In addition, compared with original PCMs, the thermal conductivity of biochar-based composite PCMs first decreased and then increased with the increase of biochar pyrolysis temperature. Moreover, biochar-based composite PCMs had better thermal stability, concerned with the dominant mechanisms of biochar encapsulated organic PCMs. Therefore, biochar-based composite PCMs proposed in this study expand the application of biochar and organic PCMs in thermal energy storage, building energy conservation, solar energy utilization, and battery thermal management system.